lint/tests: Give nice error message for common import failures.

This should make users much more likely to be able to debug issues
where they ran Zulip outside the Vagrant environment or virtualenv.

[error messages tweaked by tabbott]

frontend_tests/run-casper

@@ -1,12 +1,18 @@
 #!/usr/bin/env python
 from __future__ import print_function
 import subprocess
-import requests
 import optparse
 import time
 import sys
 import os
 import glob
+try:
+    import requests
+except ImportError as e:
+    print("ImportError: {}".format(e))
+    print("You need to run the Zulip tests inside a Zulip dev environment.")
+    print("If you are using Vagrant, you can `vagrant ssh` to enter the Vagrant guest.")
+    sys.exit(1)
 
 #
 # In order to use remote casperjs debugging, pass the --remote-debug flag

tools/lint-all

@@ -7,8 +7,13 @@ import sys
 import optparse
 import subprocess
 import traceback
-
-import lister
+try:
+    import lister
+except ImportError as e:
+    print("ImportError: {}".format(e))
+    print("You need to run the Zulip linters inside a Zulip dev environment.")
+    print("If you are using Vagrant, you can `vagrant ssh` to enter the Vagrant guest.")
+    sys.exit(1)
 
 parser = optparse.OptionParser()
 parser.add_option('--full',

tools/test-backend

@@ -6,9 +6,15 @@ import os
 import sys
 import subprocess
 
-import django
-from django.conf import settings
-from django.test.utils import get_runner
+try:
+    import django
+    from django.conf import settings
+    from django.test.utils import get_runner
+except ImportError as e:
+    print("ImportError: {}".format(e))
+    print("You need to run the Zulip tests inside a Zulip dev environment.")
+    print("If you are using Vagrant, you can `vagrant ssh` to enter the Vagrant guest.")
+    sys.exit(1)
 
 if __name__ == "__main__":
     TOOLS_DIR = os.path.dirname(os.path.abspath(__file__))